Atmospheric pressure ionization (API) used for taking out a target component as an ion in a gas phase includes electro-spray ionization (ESI) that generates ions in the process of spraying a solution containing ions into an electric field and volatilizing droplets of the solution. The ESI is broadly utilized as the interface for introducing to the MS the liquid separated by a separation analyzer, such as capillary electrophoresis (CE), liquid chromatography (LC), and ion chromatography (IC) devices (see Non Patent Literature 1).
As illustrated in FIG. 1, the ESI is to spray into an ion source 10 a liquid flow that is introduced from a capillary 8 of the separation analyzer into a nebulizer 12 of the ion source 10 to generate ions 18, and to introduce the ions 18 to a capillary 22 of an MS 20. That is, the sample solution is introduced to the capillary 8 of the ion source 10, and with an electric field applied to the sample solution, the nebulizer 12 feeds and sprays an atomized gas (also called nebulizer gas) from the outside of the capillary 8 to form fine charged droplets. As the charged droplets move, evaporation of solvent and increase in the surface electric field of the charged droplets progress. When the repulsive force between electric charges surpasses the surface tension of the charged droplets, the charged droplets are split. Through repeated evaporation and split, the charged droplets become fine droplets. In the end, the droplets are discharged into the gas phase as sample ions 18 (ion evaporation).
The electric field is generated by forming an electric potential difference between the nebulizer 12 and an ion introduction port of the capillary (inlet port 22A of the capillary 22) of the MS 20. Depending on the manufacturers, some MSs are configured such that the nebulizer side of the electrospray ion source is grounded (Patent Literature 1), while other MSs are configured such that a voltage is applied to the nebulizer side (Non Patent Literature 1).